Rigid foams and processes for their production are well known in the art. Such foams are typically produced by reacting a polyisocyanate with an isocyanate reactive material such as polyol in the presence of a blowing agent.
In recent years, the substantial increases in costs of the basic materials used to make foam, has encouraged the development and use of filler materials to reduce the amount of the basic materials used and the weight of the finished materials. One of the suggested filler materials and insulating materials utilizes hollow microspheres.
The use of hollow microspheres in foam is known in the art. The use of microspheres, however, in a syntactic foam having a bimodal cell structure is neither disclosed nor suggested by the art.
The expression "syntactic" as used herein refers to the use of hollow spheres in a polymer matrix to produce a cellular material.
The expression "PUR/PIR foam" refers to polyurethane and/or isocyanurate foam produced by generation of gas bubbles during reaction of the polymer matrix.
The expression "hybrid" as used herein refers to the simultaneous use of hollow spheres and PUR/PIR foam to produce a syntactic foam.
The expression "bimodal cell size" refers to a hybrid foam wherein the hollow spheres have a median diameter of about twice that of the cell diameter of the foamed matrix.
Expanded microspheres consisting of a synthetic thermoplastic resin shell that encapsulates a liquid blowing agent are known. See e.g., U.S. Pat. Nos. 4,829,094, 4,843,104 and 4,902,722.
In Melber, et al. (U.S. Pat. Nos. 4,829,094 and 4,843,104), a syntactic-polymer foam composition having a low density filler containing free flowing microspheres is disclosed. Melber, et al., however, does not disclose or suggest a bimodal cell structured foam.
Otloski, et al. (U.S. Pat. No. 4,916,173) discloses a polyurethane composition for a millable modeling stock application having hollow microspheres. Otloski, et al., however, discloses a solid polymer matrix and does not disclose a bimodal cell structure. Janda (U.S. Pat. No. 4,959,395) also discloses a solid polymer matrix and not a foam with a bimodal cell structure.
The two patents to Torobin (U.S. Pat. Nos. 4,303,729 and 4,303,736) disclose the use of hollow plastic microspheres as filler materials in plastics. Each of these patents does not disclose the bimodal cell structure. Additionally, the Torobin patents disclose large diameter microspheres in the range of 200 to 10,000 microns.
Cravens (U.S. Pat. No. 4,038,238) discloses hollow microspheres having lower loadings of 2 to 5 percent by weight of the total composition. Additionally, Cravens does not disclose a foam having a bimodal cell structure.
Harper (U.S. Pat. No. 4,082,702) discloses a rigid syntactic foam comprising glass microballoons. Harper, however, does not disclose a foam having a bimodal cell structure.
It has now been found that the use of hollow microspheres in a syntactic PUR/PIR foam having a bimodal cell structure results in improved compressive strength, additionally, the foams have reduced k-factor aging losses and flammability since the blowing agent is encapsulated in a microsphere.